This proposal focuses on penile acquisition of pathogenic SIVsmm infection since this transmission route accounts for nearly half of the global HIV-1 pandemic and has been understudied in comparison with other mucosal routes of transmission. Elucidation of the bottleneck to SIV/HIV transmission and the molecular and cellular mechanisms underlying successful infection at epithelial and mucosal surfaces of the penis could be instrumental in the design of effective vaccines. Our laboratories have taken significant steps toward addressing these goals by developing and validating an experimental strategy with which to identify, enumerate, and molecularly clone transmitted/founder HIV-1 and SIV genomes that are responsible for productive infection (Keele 2008; Keele 2009; Salazar 2009; Stone 2010; Bar 2010; Li 2010a,b). In this project, we will test the following hypothesis: Genetically-diverse, rhesus macaque-adapted, pathogenic SIVsmm viruses will productively infect Indian rhesus macaques by atraumatic penile inoculation and lead to persistent replication and pathogenicity. Analysis of transmitted/founder viruses in this transmission model will reveal the extent of the mucosal bottleneck and the impact of neutralizing and binding antibodies on blocking infection. A test of this hypothesis will provide mechanistic insights into the biology of penile SIV/HIV acquisition, the high transmissibility of SIV/HIV during the acute infection period, and the relative protective effect of anti-SIV/HIV antibodies in chronic infection on forward transmission. Specific aims are: (i) To productively infect Indian rhesus macaques by atraumatic penile inoculation of genetically-divergent, macaque-adapted, pathogenic SIVsmm viruses from acute infection plasma; (ii) To identify by single genome amplification and to molecularly clone transmitted/founder viruses from these animals; (iii) To determine if plasma from acutely infected animals contains virus that is more infectious and transmissible on a per virion basis than plasma from the same animals in chronic phase infection; (iv) To characterize quantitatively the bottleneck to penile transmission compared with intravenous inoculation; and (v) To determine if chronic phase plasma contains SIV-specific antibodies that prevent penile or intravenous virus acquisition. A key deliverable of this project, in addition to new scientific insights into penile transmission and mechanisms underlying 'acute- to-acute' infection, will be 8 new molecular clones of transmitted/founder viruses resulting from penile acquisition that represent widely divergent SIVsmm genetic lineages for use as potential vaccine challenge stocks and in further studies of SIV transmission biology.